The present invention relates to a digital signal reproducing apparatus being adaptable to a use of Viterbi decoding such as video tape recorders and optical disk drives. More particularly, the invention relates to a digital signal reproducing apparatus wherein equalizing references of a differential system and an integrating system are added in a predetermined ratio for decisions of maximum likelihood, or the distances (i.e., branch metrics) from respective amplitude reference values based on the equalizing references of the differential system and an integrating system are added in a predetermined ratio for a binary discrimination, thereby to improve the discriminating accuracy degraded by noise.
In Video tape recorders, optical disk drives or like, a digital signal recorded at a high density is conventionally intended to ensure reliable the reproduction thereof by processing the reproduced signal with Viterbi decoding.
In other words, Viterbi decoding defines xe2x80x9cnxe2x80x9d states determined by intersymbolic interference, through the use of combinations of the instant preceding input data. Every time the input data are changed, the current xe2x80x9cnxe2x80x9d states are replaced with the ensuing xe2x80x9cnxe2x80x9d states to process the changed input data. Specifically, when the intersymbolic interference has a length of xe2x80x9cmxe2x80x9d, the xe2x80x9cnxe2x80x9d states are determinedby the preceding (mxe2x88x921) bits. For example, when the input signal is a digital signal of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d, there exists a total of n=2(mxe2x88x921) states.
It is assumed with reference to the xe2x80x9cnxe2x80x9d states defined above that the level of noise contained in the reproduced signal has a Gaussian distribution and that the value of the reproduced signals corresponding to each noise-free state is regarded as an amplitude reference value. In that case, the likelihood of a transition to each of the xe2x80x9cnxe2x80x9d states is represented by a value obtained by squaring the difference between an amplitude reference value and an actually reproduced signal and by accumulating the squared values until one of the states is reached. According to the Viterbi decoding, the likelihood values are accumulated for each of possible paths leading from the preceding xe2x80x9cnxe2x80x9d states to each of the current states. Given the result of the calculations, it is judged that a transition has taken place over the path having the strongest likelihood (i.e., with the smallest accumulated value). With the judgment made, the current xe2x80x9cnxe2x80x9d states are replaced by the ensuing xe2x80x9cnxe2x80x9d states, and the hysteresis and likelihood of a discriminated value in each state are updated.
The transitions of maximum likelihood states are detected successively up to a stage where hysteretic records going back several bits in time are merged into a single item of hysteresis. This finalizes the result of signal discrimination so far. As outlined, Viterbi decoding discriminates the reproduced signal by making the most of the signal power of the reproduced signal where the noise superimposed on the reproduced signal is random noise. The Viterbi decoding provides an appreciable improvement of the error rate over the conventional decoding method by which the reproduced signal is compared with a predetermined threshold value for each bit.
Such Viterbi decoding is commonly used to process signals equalized in partial response. Depending on the characteristics of a transmission system in use, either the equalized characteristic of an integrating system such as PR (1, 1; referred to as PR1 hereinafter) or the equalized characteristic of a differential system such as EPR (Extended Partial Response; 1, 1, xe2x88x921, xe2x88x921; referred to as EPR4 hereinafter) is adapted to partial response equalization.
FIG. 18 is a table showing state transitions of a combination of RLL (Run Length Limited; 1, 7) code with EPR4 equalization. The RLL (1, 7) code is a coding method whereby at least two logical 1s or 0s always occur continuously (a single logical 1 or 0 will not occur under the coding scheme based on what is known as d=1 restriction). EPR4 entails intersymbolic interference in subsequent three bits for each input data item because of PR (1, 1, xe2x88x921, xe2x88x921).
In the above combination, the hysteresis of input data of up to three earlier bits determines uniquely the state transition (output) of the subsequently input data. In FIG. 18, a[k] denotes input data, and a[kxe2x88x921], a[kxe2x88x922] and a[kxe2x88x923] stand for input data which are one, two and three clock pulses previous to the input data a[k] respectively. A state b[kxe2x88x921] resulting from the input data a[kxe2x88x921], a[kxe2x88x922] and a[kxe2x88x923] is represented by a symbol S together with respective values of the input data a[kxe2x88x921], a[kxe2x88x922] and a[kxe2x88x923]. For example, when the input a[k] has a value of 0 in a state (S000), then an output c[k] with a value of 0 is obtained, and a state b[k] is changed to (S000).
According to the RLL (1, 7) code, the states (S010) and (S101) do not occur under the d=1 restriction. With the two states (S010) and (S101) excluded, each state b[kxe2x88x921] is changed to two states corresponding to the 0 or 1 input, whereby six states are taken as a whole. In the case of the RLL (1, 7) code, the output signal c[k] has five amplitude reference values: xe2x88x922, xe2x88x921, 0, 1 and 2. These relations are illustrated in a trellis diagram of FIG. 19.
As shown in the trellis diagram of FIG. 19 formed by repetitive patterns, the Viterbi decoding method decodes the input signal by accumulating squared values of the difference between an EPR4 equalized reproduced signal and an EPR4 equalized amplitude reference value (the difference is made of distances, i.e., branch metrics) and by selecting the path having the smallest accumulated value (metric).
With regard to the equalized characteristic of an integrating system such as PR1, a low frequency region tends to be emphasized excessively as shown in FIG. 20. When the equalized characteristic of such an integrating system is applied to a magnetic recording and reproducing system having difficulty in reproducing DC components, the low frequency region of the latter system is likely to be inordinately emphasized. This results in a deterioration of the accuracy of signal discrimination due to low frequency noise such as cross talk.
With respect to the equalized characteristic of a differential system such as EPR4, the low frequency region is suppressed while a high frequency region with an inferior S/N ratio tends to be emphasized, as illustrated in FIG. 21. when the equalized characteristic of such a differential system is applied to a magnetic recording and reproducing system for high-density (short waveform) recording, the presence of high frequency noise can make it difficult to ensure sufficient accuracy of signal discrimination.
It is appreciated that recording density, for example will be further enhanced when such noise-degraded levels of accuracy in signal discrimination are improved.
Therefore, the present invention is invented to overcome the above deficiencies and disadvantages of the prior art and it is the object thereof to provide a digital signal reproducing apparatus capable of improving noise-degraded levels of accuracy in signal discrimination.
According to the present invention, there is provided a digital signal reproducing apparatus wherein equalized signals of a differential system and an integrating system are added in a weighted manner to obtain an added equalized signal which is subjected to maximum likelihood decoding, whereby a result of binary discrimination corresponding to an input signal is outputted.
Metrics of the differential system and the integrating system are added using a predetermined weighting factor every transitions corresponding thereto. Then, the calculated metrics are accumulated to obtain likelihood values for each of transition paths. Next, the strongest likelihood is judged from the obtained values, and a discriminated binary value is outputted corresponding to the input signal.
When the transition of the integrating system reaches a predetermined equalizing reference value during the weighted adding process above, additions are made selectively for the transitions of the equalizing reference value in question.
In the above cases, the weighting factor is modified in accordance with detected result of the level fluctuations of the input signal.
The equalized signals of the differential system and the integrating system are added up in a weighted fashion to generate an added equalized signal so that, by equalizing frequency characteristics of noise components, this process may generate an added equalized signal which, with its noise characteristics close to those of the white noise, is best fit for Viterbi decoding. When the added equalized signal is subjected to maximum likelihood decoding in order to output a discriminated binary value corresponding to the input signal, the noise-induced degradation of signal discrimination accuracy can be effectively prevented.
The metrics of the integrating system and the differential system are added using a predetermined weighting factor every transition corresponding thereto, and then the added metrics are accumulated to obtain the likelihood of each transition. This provides likelihood levels in the same manner as in the case where frequency characteristics of noise components are equalized so as to generate an added equalized signal that, with its noise characteristics close to those of the white noise, is best fit for Viterbi decoding. As a result, binary discriminated values are outputted in such a way that effectively averts noise-induced degradation of accuracy in signal discrimination.
When the transition of the integrating system reaches a predetermined equalizing reference value during the weighted adding process, additions may be made selectively for the transitions of the equalizing reference value in question. In such a case, the selective additions may be illustratively timed so that the equalizing reference value is set to zero. This makes it possible to avoid lowering discriminatory levels for the input signal caused by the latter""s amplitude fluctuations.
In any of the above cases, the weighting factors are varied in accordance with the detected result of the level fluctuations of the input signal. This permits effective removal of those frequency characteristic fluctuations in noise components, which accompany amplitude fluctuations of the input signal. Thereby, eliminating the frequency characteristic fluctuations in turn may avert degradation of signal discrimination accuracy caused by the amplitude fluctuations.